Minor clean-ups.

* Track the length of the dex file in RawDexFile.  This can be used to
  improve its assertions.

* Move a sanity check for MmapCloser into the constructor.

* Add a TODO for moving the base64 openers into test code.

* Make the base64 decoder assume C strings, mention this in a comment.

Change-Id: Ib4d5a43d23ebe8819032763c42c536c8b826723a
diff --git a/src/base64.cc b/src/base64.cc
index e745a62..7c09688 100644
--- a/src/base64.cc
+++ b/src/base64.cc
@@ -32,11 +32,11 @@
   255, 255, 255, 255
 };
 
-byte* DecodeBase64(const char* src, size_t size, size_t* dst_size) {
+byte* DecodeBase64(const char* src, size_t* dst_size) {
   std::vector<byte> tmp;
   unsigned long t = 0, y = 0;
   int g = 3;
-  for (size_t i = 0; i < size; ++i) {
+  for (size_t i = 0; src[i] != '\0'; ++i) {
     byte c = kMap[src[i] & 0xFF];
     if (c == 255) continue;
     // the final = symbols are read and used to trim the remaining bytes